One of the most important, yield-reducing diseases in corn is gray leaf spot (GLS), primarily caused by Cercospora zeae-maydis (Cz) Tehon & E. Y. Daniels (reviewed by Ward et al. 1999 Plant Dis. 83:884-895). GLS is a global problem and, in addition to prevalence in Africa, Central America and South America, it has spread across most of the U.S. cornbelt over the past 10-15 years. The fungus overwinters in field debris and requires moisture, usually in the form of heavy fogs, dews, or rains, to spread its spores and infect corn. Increasing pervasiveness has been linked to no-till practices which promote retention of fungi, such as Cz, in the soil (Paul et al. 2005 Phytopathology 95:388-396). Symptoms include a rectangular necrotic lesion which can coalesce to larger affected regions and symptoms usually appear later in the growing season. GLS in corn elicits an increased allocation of resources to damaged leaf tissue, leading to elevated risk for root and stalk rots, which ultimately results in even greater crop losses (Ward et al. 1999; Saghai-Maroof et al. 1996 Theor. Appl. Genet. 93:539-546). Yield-loss associated with GLS can be high if the symptoms are heavy and appear early, with reported losses exceeding 50% (Ward et al. 1999). Further, even if crop management strategies, such as fungicide application, are employed to reduce the incidence of Cz in the soil, there is still risk of acquiring infection from proximate fields. Notably, Cz can be readily dispersed by wind (Latterell et al. 1983 Plant Dis. 67:842-847). Thus there is a substantial need for the development of GLS resistant corn.
The introgression of disease resistance into elite germplasm has been enhanced by the advent of molecular marker-assisted breeding, which has not only dramatically increased genetic gain in agronomic traits but has also led to the identification of marker-trait associations for secondary traits. The efficacy of this approach for disease resistance breeding in maize was recently reviewed by Wisser et al. (Wisser et al. 2006 Phytopathology 96:120-129). This review also highlighted the lack of genetic resolution in many of these reports and called into question the accuracy of many historical disease resistance mapping studies due to inadequate sampling and mapping population inadequacies. In general, disease resistance mapping is difficult due to the inconsistencies of pathogen infection that can occur in field trials. In addition, the screening of materials only in summer nurseries due to regulations restricting the use of pathogens and the economics of screening for pathogens in winter nurseries make screening for disease resistance a difficult task.
Moreover, recent work has identified there are at least two sister species of Cz, as well as potentially other isolates of Cercospora, capable of causing GLS (Carson et al. 2006 Maydica 51:89-92; Carson et al. 2002 Plant Dis. 86:1088-109). Because different races have distinct epidemiologies, this has bearing on the methodology of GLS phenotyping used as the basis for these mapping studies, bringing into question the very nature of many so-called GLS resistance QTL.
The present invention provides and includes a method for screening and selecting a corn plant comprising QTL for GLS resistance that were derived from Brazilian mapping populations using endemic strains of Cz and single nucleotide polymorphisms (SNP) marker technology.